Electrical switches and circuits therefor

ABSTRACT

Elements of metal oxide varistor material are integrally incorporated into the assemblies of switches such as breaker point assemblies and electric door bell assemblies to provide the dual functions of insulating the contact element supporting parts thereof and for limiting the voltages developed across the contact elements thereby minimizing arcing across the contact elements and consequent erosion thereof and electromagnetic radiation produced thereby.

I United States Patent [1 1 1111 3,710,061 Harnden, Jr. 1 51 Jan. 9, 1973 s41 ELECTRICAL SWITCHES AND 2,854,534 9/1958 Beauclar ..200 27 A CIRCUITS THEREFOR 3,603,758 9/1971 Fry etal. ....200/19A Inventor: J D. Hamden J Schenectady, 3,646,30l 2/1972 Huntzlnger et al. ..200/19 A v Primary Examiner-l-l. 0. Jones [73] Assignee: General Electric Company Attorney-John Ahern et al.

22 Fl (1: D .22 1971 I 1 Y e cc 57 ABSTRACT [211 App]. No.: 211,008

Elements of metal ox1de var1stor matenal are Integrally incorporated into the assemblies of switches [52] US. Cl. ..200/166 C, 200/19 A, 200/27 A, such as breaker point assemblies and electric door bell zoo/31 A assemblies to rovide the dual functions of insulating 51 1 t Cl 110111 1 02 p I 1 .f h Contact element pp g Parts thereof and v [5 8] 0 Search "zoo/l9 27 31 166 C limiting the voltages developed across the contact ele- References Cited ments thereby m1n1m1z1ng arcing across the contact 7 I elements and consequent erosion thereof and elec- UNITED STATES PATENTS tromagnetic radiation produced thereby.

3,221,114 11/1965 Maeda ..200/166 C 15 Claims, 19 Drawing Figures PATENTEI] JMI 9 I978 SHEET 1 OF 3 PATENTEDJAN 9 I973 3.710.061

sum 3 [IF 3 ELECTRICAL SWITCHES AND CIRCUITS THEREFOR The present invention relates in general to electrical switches and in particular to electrical switches for use in vibratory systems in which the contacts thereof are vibrated to make and break electrical circuit connectrons.

Such vibratory systems are utilized to provide in a number of ways, for example, ignition in internal combustion engines and to provide mechanical vibratory feet the timing of the spark discharges in the high tension or secondary circuits of the ignition system.

According, an object of the present inventionis to provide improvements in electrical switches and circuits therefor.

Another object of the present invention is to provide an electrical switch having contact elements of conventional materials yet which have longer life than when used in conventional switches.

Another object of the present invention is to provide a vibratory switch assembly which provides superior performance.

Another object of the present invention is to reduce the number of parts utilized in a vibratory switch assembly.

A further object of the present invention is to provide a vibratory switch assembly of reduced size.

In carrying out the invention in anillustrative embodiment thereof, there is provided a pair of conductive members on which are located a pair of contact elements, each contact element being secured to a respective one of the conductive members. The conductive members are pivotally supported about an axis so that at least one of the members is pivotal thereabout with respect to the other of the conductive members. The contact elements on the members are aligned to engage and disengage in response to movement of the movable conductive member. A pair of terminal elements is provided. Each of the conductive members is connected to a respective one of the terminal elements. An element of metal oxide varistor material having a pair of surface portions is connected in circuit with the pair of terminal elements. The metal oxide varistor material has an alpha in excess of 10 in the current density range of l()"" to 10 amperes per square centimeter. The surface portions of the metal oxide varistor element are spaced to provide a standby current flow between the terminals which is low when normal operating voltage appears across the terminals and when voltages in excess of normal voltage progressively appear thereacross, rapidly decreasing impedance is presented by the elements in accordance with the alpha of the metal oxidevaristor material, thereby limiting the amplitude of the voltage between the electrodes.

The features of the present invention which are beto its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description takenin connection with the accompanying drawings in which,

FIG. 1 is a schematic diagram of the ignition system of an internal combustion engine,

FIG. 2 is a perspective view of a breaker point assembly such as is conventionally used in the ignition systems of internal combustion engines,

FIG. 3 is a plan view of the assembly of FIG. 2 showing a modification of the breaker point assembly thereof in accordance with the present invention,

FIG. 4 shows graphs of the electrical characteristics of the three material of differing voltage gradients and alphas suitable for use in the switch devices of. the present invention,

FIG. 5 is a plan view of another embodiment of the present invention,

FIG. 6A is a plan view of a modification of the assembly of FIG. 2 in accordance with another embodiment of the present invention,

FIG. 6B is an end view of the assembly of FIG. 6A,

FIG. 7A is a plan view of another modification of the breaker point assembly of FIG. 2 in accordance with another embodiment of the present invention,

FIG. 7B is an end view of the embodiment of FIG. 7A,

FIG. 8A shows another modification of the assembly of FIG. 2 in accordance with another embodiment of the present invention,

FIG. 8B is an end view of the embodiment of FIG. 8A,

FIG. 9 shows a plan view of a modification of the assembly of FIG. 2 in accordance with a further embodiment of the present invention,

FIG. 9B is an end view of the embodiment of FIG.

FIG. 9C is a rear view of the embodiment of FIG. 9A,

FIG. 10 is a plan view of a conventional electrical bell employing a vibrating switch assembly,

FIG. 11 is a schematic diagram of the electrical circuit of the electric bell of FIG. 10 incorporating a modification in accordance with the present invention,

FIG. 12A shows a plan view of another modification of the bell of FIG. 10in accordance with the present invention,

FIG. 12B is a sectional view of the embodiment shown in FIG. 12A, 2

FIG. 13 is a schematic diagram of the electrical circuit of the bell of FIG. 10 incorporating a further modification in accordance with the present invention.

Referring now to FIG. 1, there is shown a schematic diagram of an ignition system of an internal combustion engine. The ignition system includes a distributor l0 usually referred to as an ignition coil, a transformer 11 having a primary winding of a small number of turns and a secondary winding of a large number of turns, a

direct current source of power in the form of a battery I 14, and utilization means in the form of a spark plug 151' The distributor 10 includes a breaker plate 16 on which is mounted a breaker point assembly 17. The breaker point assembly 17 includes a base member 20 on which eight terminal is mounted a breaker arm 21 to one end of which is secured a contact element 18 and which is pivotally mounted at the other end thereof to the base member 20. Another conductive member 22 with a contact element 24 is mounted on the base member 20. The base member 20 of the breaker point assembly is secured in place on the breaker plate 16 by means of screws passing through holes in the base member. The holes may be in slotted form to permit adjustment of the base member in respect to a cam member 23 mechanically geared to the crankshaft of the engine. Breaker plate 16 is grounded and consequently the portion of the base member contacting the breaker plate may be considered the terminal element or means for the conductive member 22. The side of the breaker arm 21 adjacent the cam 23 is provided with an insulating rider or ridge member 25 which engages the cam. Rotation of the cam 23 causes the contacts 18 and 24 to periodically engage and disengage. As the cam is octagonal in form, the ignition system shown is suitable for use with an internal combustion engine having eight cylinders. Consequently, for each revolution of the cam, the breaker point switch operates through eight cycles of engaging and disengaging of contacts. The positive terminal of the battery 14 is connected to ground. The negative terminal of the battery is connected through an ignition switch 30 to one end of the primary winding 12 of the ignition coil. The other end of the primary winding of the ignition coil is connected to a terminal element conductively connected to the arm member 21. A capacitor 26 is connected between arm member 21 and ground. The fixed contact 24 of the breaker arm of the assembly is connected through the breaker plate to ground. One end of the secondary winding 13 of the ignition coil is connected to ground and the other end is connected to the rotor 31 of the distributor. The rotor cap 32 of the distributor assembly is provided with a plurality of contacts each corresponding to a respective spark plug. Only one such contact 33 is shown and also only one such spark plug is shown. For an engine with eight cylinders, the cap would be provided with points equally spaced about the periphery of the cap.

ln the operation of the ignition system with the ignition switch 30 closed, the engine operating, the cam member 23 rotates and actuates of the switch 17. Upon opening of the contacts 18 and 24 of the switch, the primary circuit of the distributor is broken causing a high voltage to be induced in ,the secondary winding 13 which is applied to thespark plug through the rotor circuit of the distributor. Coincidentally, with the breaking of contacts 18 and 24 in the primary circuit, the

rotor 31 is aligned with the appropriate contact element connecting it to the spark plug of the cylinder to be fired. The repeated operation of the contact elements of the breaker point switch causes them to wear. The breaking of electrical contact in the primary circuit produces sparking across the contacts which not only effects the proper timing of the spark developed in the spark plug but also over a period of time, causes a wear and a deterioration of the contact surfaces of the contact elements. To overcome such problems, a capacitor heretofore has been connected in shunt with the contacts to absorb the inductive currents produced on disengagement of contact elements in an attempt to bypass the contacts. Even with such a capacitor, high voltages exist and considerable arcing occurs at the contacts resulting in constant wear thereof. The present invention is directed to minimizing such problems.

Reference is now made to FIG. 2 which is an enlarged perspective view of a conventional breaker point assembly such as the breaker point assembly described and shown in FIG. 1. The breaker point assembly includes a base member having a pair of holes, each of which is slotted in form so as to permit adjustment thereof with respect to the actuating cam 23 for setting the timing of disengagement of the contact elements thereof. On the base member 40 is securely mounted a shaft 43 extending normal to the surface of the base member. An arm member 44 includes at one end a nylon hub 45 pivotally connected to the shaft 43 and a conductive arm member 46 with a contact element 52 at the other end. Also, secured to the nylon hub member and conductively connected to conductive member 46 by rivets extending through the hub 45 is a spring member 47, the other end of which is connected to a terminal element 48 or lug supported by spring 47 against an insulating member 49 which in turn is sup ported by tab 50 on base member 40. Also provided on a base member 40 is another conductive member 51 extending upright from the base member and provided with a contact element 53 registering with the contact element on conductive member 46. The contact elements on conductive member 46 and 51 are aligned to engage and disengage in response to movement of the conductive member 46 about the axis of the shaft 43.

Reference is now made to FIG. 3 which shows an improvement in the breaker point assembly of FIG. 2. In this figure, elements identical to the elements of FIG. 2 are designated by the same numerals. In FIG. 3, the hub member 55 is formed in two sections 56 and 57. The section 56 adjacent the contact element 52 is of nylon to provide good wear of the rider element 58 thereof. The section 57 is constituted of metal oxide varistor material. On end section 59 of spring 47 is riveted to member 46 through both sections 56 and 57 to provide an integral breaker arm unit. The cylindrical aperture portion and the adjacent external surface of the hub member 57 may be metallized, to such metallizations being designated 61 and 62 to obtain better control over the voltage versus current characteristics of the member 57 of metal oxide varistor material as will be explained in a connection with FIG. 5. Metallization 62 is in conductive contact with spring member 47.

The hub member 57 is constituted of a metal oxide varistor material such as described in Canadian patent 831,691, which has a nonlinear voltage versus current characteristic. The metal oxide varistor material described in the aforementioned patent is constituted of fine particles of zinc oxide with certain additives which have been pressed and sintered at high temperatures to provide a composite body or wafer of material. The current versus voltage characteristics of the composite body is expressed by the following equation:

I /C) l where V is voltage applied across a pair of opposed surfaces or planes,

1 is the current which flows between the surfaces,

C is a constant which is a function of the physical dimensions of the body as well as its composition and the process used in making it,

a is a constant for a given range of current and is a measure of the nonlinearity of the current versus voltage characteristic of the body.

In equation I, when V is used to denote voltage between opposed planes of a unit volume of material, or voltage gradient, current flow through the unit volume of material in response to the voltage gradient material is 0.12 calories per degree Centigrade per gram. Accordingly, on this account, as well, heat absorption capability of the material is advantageous as a surge absorption material.

becomes current density. For the metal oxide varistor material for current densities which are very low, for example, in the vicinity of a microampere per square centimeter, the alpha a is relatively low, i.e., less than 10. In the current density range of from to 10 amperes per square centimeter, the alpha is high, i.e., substantially greater than 10 and relatively constant. In the current density ranges progressively in excess of 10 amperes per square centimeter, the alpha progressively decreases. When the current versus voltage characteristic is plotted on log-log coordinates, the alpha is represented by the reciprocal of the slope of the graph in which current density is represented by the abscissa and voltage gradient is represented by the ordinate of the graph. For a central range of current densities of from 10 to 10 amperes per square centimeter, the reciprocal of the slope is relatively constant. For current densities below this range, the reciprocal of the slope of the graph progressively decreases. Also, for current densities above this range, the reciprocal of the slope of the graph progressively decreases.

The voltage gradient versus current density characteristics of three types of material in log-log coordinates are set forth in FIG. 4. Graphs 34 and 35 are materials of high voltage gradient material and graph 36 is a graph of low voltage gradient material. For all of the graphs in the current density range from 10' to 10 amperes per square centimeter, the alpha is high and is substantially greater than 10 and relatively constant. For current densities progressively greater than 10 amperes per square centimeter, the alpha progressively decreases. For current densities progressively less than 10" amperes per square centimeter, the alpha also progressively decreases;

As the metal oxide varistor material is a ceramic material, the surfaces thereof may be metallized for facilitating electrical connections thereto in a manner similar to the manner in which other ceramic materials are metallized. For example, Silver Glass Frit, du Pont No. 7713, made by the du Pont Chemical Company of Wilmington, Delaware, may be used. Such material is applied as a slurry in a silk screening operation and fired at about 550C to provide a conductive coating on the surface. Other methods such as electroplating or metal spraying could be used as well.

The nonlinear characteristics of the material results from bulk phenomenon and is bi-directional. The response of the material to steep voltage Wave fronts is very rapid. Accordingly, the voltage limiting effect of the material is practically instantaneous. Heat generation occurs throughout the body of material and does not occur in specific regions thereof as in semiconductor junction devices, for example. Accordingly, the material has good heat absorption capability as the conversion of electrical. to thermal energy occurs throughout the material. The specific heat of the The material, in addition to the desired electrical and thermal characteristics described above, has highly desirable mechanical properties. The material has a fine grain structure, may be readily machined to a smooth surface and formed into any desired shape having excellent compressive strength. The material is readily molded in .the process of making it. Accordingly, any size or shape of material may be readily formed for the purposes desired.

For the breaker point assembly of FIG. 3, the member 57 is constituted of metal oxide varistor material of the particular voltage gradient versus current density characteristics and the radial spacing between the conductive portions 61 and 62 is set to provide the desired voltage versus current characteristics for the member 57. For a nominally six volt ignition system, such as shown in FIG. 1, the geometry of the metal oxide varistor material of member 57 and its constitution is selected to provide high impedance at nominally 6 volts, i.e., to provide low standby current flow when normal operating voltage appears thereacross and when voltages in excess of normal operating voltage progressively appear across the conductive portions 61 and 62 and hence between contacts 52 and 53, a rapidly decreasing impedance is presented by member 57 in accordance with the alpha of the metal oxide varistor material of which it is constituted. Accordingly, the voltage between contacts 52 and 53 is limited to low values with resultant prolongation of the life of the contacts, improvement of the performance of the contacts, and reduction substantially of the electromagnetic radiation produced by sparking at the contacts.

Reference is now made to FIG. 5 which shows a plan view of another conventional breaker point assembly of standard design utilized in conventional automobile ignition systems. In this design, a conventional insulative element is replaced with an element 65 of metal oxide varistor material to perform a protective function of limiting the voltage excursions between the contact elements of the assembly as well as an insulative function. The assembly includes a first base member having a pair of slots located at opposite ends thereof for attachment of the base member to the base plate of a dis tributor such as base plate 16 of FIG. 1. A pair of upright support members 71 and 72 integral with the base member are provided, one for retention of an adjusting screw 73 and the other for the support of the element 65 and a spring member 74. The assembly also includes a second base member 75, one end of which is pivotally mounted to a shaft 76 extending upright from the base of the member 70. The second base member includes at the end thereof opposite the portion engaging the shaft 76 end thereof a slot 77 and a pair of upright portions 78 and 79. A fastener member 80 secured in slot 77 permits integral movement of the second base member along the surface of the first base member 70 about the shaft 76 and at the same time, maintains the second base member 75 in contact with the first base member. The upright portion 79 is provided with a threaded hole which is adapted to receive screw 73 which also extends through the upright portion in the first base member, thereby permitting movement of the second base member 75 about the axis of shaft 76. The other upright member 78 of the second base member 75 is provided with a conductive contact 81 or point. A conductive breaker arm 82 is provided having a cylindrical opening at one end thereof in which is mounted a cylindrical bushing of insulating material 83, the inner surface of which engages the shaft 76 and is movable thereabout. The other end of the breaker arm 82 is provided with a contact 84 or point which registers with contact 81 of the first conductive member. Spring member 74 is provided conductively connected to the arm 82 intermediate the ends thereof and the other end conductively connected to one surface portion of the insulating member 65, the other surface portion of which bears against the upright member 72. A forked lug 48 with lead attached is sandwiched between the spring member 74 and the member 65 to provide a ungrounded terminal for the breaker point assembly, the grounded terminal of which is the base member 70. The spring 74 is biased so that contacts normally are in engagement. The movable arm is provided with a riser element 85 of insulating material riveted between the member 82 and the spring 74 which in the assemblied distributor engages a cam such as cam 23 of FIG. 1. Accordingly, the switch is adapted to be cycled through a cycle of engagement and disengagement by movement of the cam. In accordance with thepresent invention, the insulating member 65 is constituted of metal oxide varistor material of particular voltage gradient versus current density characteristics and a geometry to provide the desired voltage versus current characteristic to provide high impedance for normal voltage between contacts and progressively lower impedance at progressively higher voltages. If desired, the insulating element 83 could be provided of metal oxide varistor material.

Reference is now made to FIGS. 6A and 6B which show plan and end views, respectively, of a modification of the distributor point assembly of FIG. 2 in accordance with another aspect of the present invention. FIGS. 6A and 6B show only those parts of the assembly which are different from the assembly set forth in FIG. 2 and elements of FIGS. 6A and 68 identical to elements'of FIG. 2 are identically designated. An element 90 of metal oxide varistor material having a pair of spaced surface portions 91 and 92 is provided. One surface portion 91 is bonded to the conductive member 22 supporting the fixed contact 24. The other surface portion 92 is provided with a conductive coating to which is secured one terminal of a flexible conductive wire 93, the other end of which is conductively connected to the conductive member 21. In the operation of the assembly .of FIGS. 6A and 6B, when the contacts 18 and 24 are separated or disengaged, the element 90 of the metal oxide varistor material is connected across the conductive members 21 and 22 and provides its voltage limiting function in accordance with its constitution and geometry as explained above in connection with FIGS. 3 and 5.

Reference is now made to FIGS. 7A and 7B which show, respectively, plan and end views of a breaker point contact assembly in accordance with another embodiment of the invention. The embodiment is identical to the embodiment of FIG. 2 except for the modification indicated in these figures. Elements of the embodiment of FIGS. 7A and 73 identical to the embodiment of FIG. 2 are identically designated. In this embodiment, contact element secured to the stationary conductive member 22 is provided with a pair of contact portions 96 and 97. The contact portion 97 is located adjacent the pivotal axis of the movable contact member and the contact portion 96 is located remote from the pivotal axis. Contact portion 97 is constituted of a metal oxide varistor material. The contact element 18 conductively secured to the movable contact member is made of conventional contact material, such as cadmium oxide or any of the alloy materials utilized for such applications. Upon disengagement of the contact elements, the contact portion 97 disengages subsequent to the disengagement of the contact portion 96. Accordingly, dissipation of inductive energy attendant upon disengagement of the contacts occurs in the form of increased current flow through the contact portion 97 constituted of metal oxide varistor material. The current path through the contacts is initially of low impedance when both contact portions 96 and 97 are in engagement with the contact 18. When the contact portion 96 is separated from the contact 18 and the contact portion 97 is still in engagement with the contact portion, high impedance is initially presented by the contact portion 97. As the current increases, the impedance is lowered thereby maintaining the voltage across the contacts to a low value while the energy is dissipated thereon by inductive current flow. When the contact portion 97 separates from the contact element 18, substantially all of the inductive energy has been dissipated in the contact portion 97 and accordingly, upon separation, the voltage across the contacts is limited to a small value and very little arcing and sparking occurs. Consequently, improved operation of the contact elements is obtained as well as longer life is achieved.

Reference is now made to FIGS. 8A and 8B which show plan and end views, respectively, of another modification of the conventional structure shown in FIG. 2. Elements of FIG. 8A and 83 identical to the elements of FIG. 2 are identically designated. Only those portions of the embodiment which are modified are shown in the FIGS. 8A and 8B. In this figure, the conductive support member 22 and the contact 24 secured thereto is identical to the structure shown in FIG. 7A. The movable conductive support member 21 is identical to the movable support member of FIG. 2. However, the contact element 100 secured to the member 21 includes a pair of contact portions 101 and 102. The portion 102 adjacent the pivotal axis of the movable member 21 is constituted of a metal oxide varistor material. The portion 101 more remote from the pivotal axis is constituted ofa conventional contact material. Upon disengagement of the contact elements, the contact portion 102 disengages subsequent to the disengagement of the contact portion 101. Accordingly, dissipation of inductive energy attendant upon disengagement of the contacts occurs in the form of increased current flow through the contact portion 102 constituted of metal oxide varistor material. The current path through the contacts is initially of low impedance when both contact portions 101 and 102 are in engagement with the contact 24. When the contact portion 101 is separated from the contact 24 and the contact portion 102 is still in engagement with the contact 24, high impedance is initially presented by the contact portion 102. As the current increases, the impedance is lowered thereby maintaining the voltage across the contacts a low value while the energy is dissipated thereon by the inductive current flow. When the contact portion 102 separates from. the contact element, substantially all of the inductive energy is dissipated in the contact portion l02 and accordingly, upon separation, the voltage across the contacts is limited to a small value and very little arcing and sparking occurs. Consequently, improved operation of the contact elements is obtained as well as longer life is achieved.

Reference is now made to FIGS. 9A, 9B and 9C which show, respectively, a plan, an end and a rear view of another embodiment of the present invention which is a modification of the structure shown in FIG. 2. Only those portions of the embodiment which is modified is shown in FIGS. 9A, 9B and 9C. The elements of the embodiments of FIGS. 9A, 9B and 9C which are identical to the assembly shown in FIG. 2 are identically designated. In this embodiment, the movable conductive support member 105 is constituted of a pair of bifurcate parts 106 and 107. A contact portion 108 of metal oxide varistor material is provided on part 107 and a contact portion 109 of conventional contact material is provided on bifurcate part 106. The contact portions 110 and 111 connected to the stationary conductive support member are constituted of conventional contact material. The contact 108 of metal oxide varistor material engages and disengages with contact portion 110 and similarly, the contact portion 109 connected to the other bifurcate part engages and disengages with the contact portion 111. The conductive support parts 106 and 107 are made of resilient material. The part 107 is biased toward the stationary support member 22 so that the contact portion 108 disengages subsequent to disengagement of the contact portion 109. Accordingly, on disengagement of the conductive member 105 from the conductive member 22, the metal oxide varistor material contact element 108 is placed in shunt with the opened contact portions 109 and 111 and thereby absorbs the inductive energy released by disengagement of the contacts. By the time the contact elements 108 and 110 are disengaged, a very substantial portion of the inductive energy has been dissipated in themetal oxide varistor material of contact 108 and consequently upon the complete disengagement of the members 22 and 105, only small voltages appear thereacross the contacts with con sequent reduction of sparking and arcing if desired, contact portions 108 and 107 could be interchanged, i.e., the contact of metal oxide varistor material could be located on the stationary member 22.

Reference is now made to FIG. which shows a plan view of a conventional electromagnetically operated bell, such as is used as door bells in which a vibratory contact assembly is utilized for acoustically exciting a bell member. The bell 115 includes a base or plate member 116 of ferromagnetic material, such as steel, at one of which-is mounted a cup-shaped vibratory or bell member 117, a hammer member 118 having a magnetic handle 119, is mounted on the base plate by means of a resilient conductive strip 120 constituted of a material such as beryllium copper. One end of the strip is conductively secured to an upright conductive support member 121 which is attached to the plate 116. One end of the handle hammer 119 is secured to an intermediate portion of the strip 120. The other end of the resilient strip 120 is provided with a contact element 122. Also mounted on another support member 123 of the base plate 116 are a pair of solenoids 124 and 125, one end of each of the cores of which is rigidly secured to the upright portion 123 of the base plate and the other end of each of the cores is juxtaposed to the magnetic handle 119 of the hammer. Also provided on the base member 116 is a stationary conductive member 114 mounted on an insulating member 126 which maintains the conductive member in insulating relationship to the base member. The insulating member 126 is mounted over an aperture in the base member 116 and a pair of tabs on opposite sides of the aperture are peaned over the upper side of the insulating member 126 to hold it in place. The conductive member 114 is provided at the base end thereof with a pair of protruding tabs which are peaned over on the under side of the insulating member 126. The other end of the conductive member 114 is provided with a contact 127 which normally engages the contact 122 on the resilient strip when the hammer 119 is unenergized. A conductive terminal 130 in the form of a metal receptacle with a threaded hole therein is mounted on another insulating member 131 disposed over another aperture in the base plate 116. Tabs on opposite sides of the aperture are peaned over the top side of the insulating member 131 to hold it in place. The receptacle 130 is provided with a pair of tabs which extend through holes in the insulating member 131 and are peaned over on the under side of the insulating member to hold the receptacle in place on the insulating member. The receptacle 130 is provided with a screw 132 for fastening a lead thereto. Another threaded aperture is provided in the base plate into which a screw 133 is threaded to provide the other terminal of the bell. The solenoids of the electromagnets 124 and are connected in series between the terminal and the stationary conductive member 114. When energizing voltage source is connected between the terminals I30 and 133, circuit is completed through the solenoids through the stationary conductive support member 114 through the movable conductive support member 120 to the other terminal. Accordingly, the hammer on the bell is actuated to produce a sound in the bell. Actuation of the hammer breaks the contacts and accordingly, the hammer returns to its energized position where Contact is again made and the cycle is repeated.

In accordance with one aspect of the present invention, as illustrated in FIG. 11, the insulating member 126 of FIG. 10, designated element 135 in FIG. 11 is constituted of a metal oxide varistor material. FIG. 11 shows a schematic diagram of the resultant electrical circuit with the indicated modification. Elements of FIG. 11 identical to elements of FIG. 10 are identically designated. The thickness of the element 135 is set so that for normal operating voltages, the impedance presented by the material is quite high, that is, it acts as an insulator. For voltages above a desired voltage, the characteristics of the material areset so that the impedance of the material is lowered substantially, thus the element provides a voltage limiting means across the contact which minimizes the sparking and arcing and the deleterious effects thereof such as wear of the contacts and the production of electromagnetic radiation. The tabs on the base plate 116 and the tabs on the conductive member 114 are so arranged that the shortest distance between any point on the conductive member and any point on the base plate is through the body 135 of the metal oxide varistor material from the top surface thereof to the opposed bottom surface thereof. Accordingly, the thickness of the material is controlling with respect to the operating characteristics of the material. In connection with the circuit diagram of FIG. 11, the metal oxide varistor element 135 is connected between the stationary contact and ground. Accordingly, when the contacts 122 and 127 are disengaged, the voltage appearing between the contacts is limited by the voltage allowed by the metal oxide varistor material.

Referring now to FIGS. 12A and 128, there is shown a modification of the bell assembly of FIG. 10. FIG. 12A shows a plan and FIG. 123 shows an end view ofa portion of the bell assembly immediately adjoining the contact elements 122 and 127 of the bell assembly. Elements of FIGS. 12A and'12B identical to elements of FIG. are identically designated. In the embodiment of FIGS. 12A and 128, the stationary conductive member 114 is provided with a disc 136 of metal oxide varistor material having a pair of opposed surfaces, one of which is secured to the back side of the conductive member 114 in conductive connection with the contact tion and geometry as explained above in connection with FIGS. 3 and 5.

In accordance with another aspect of the present invention, as illustrated in FIG. 13, the insulating member 131 of FIG. 10, designated element 140 in FIG. 13'is constituted of a metal oxide varistor material. FIG. 13 shows a schematic diagram of the resultant electrical circuit with the indicated modification. Elements of FIG. 13 identical to elements of FIG. 10 are identically designated. The thickness of the element 140 is set so that for normal operating voltages, the impedance presented by the material is quite high, that is, it acts as an insulator. For voltages above a desired voltage, the characteristics of the material are set so that the impedance of the material is lowered substantially, thus the element provides a voltage limiting means across the contacts which minimizes the sparking and arcing and the deleterious effects thereof such as wear of the contacts and the production of electromagnetic radiation.

While the invention has been described in specific embodiments, it will be appreciated that modifications may be made by those skilled in the art and I intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A switch assembly comprising a base member,

a pair of conductive electrical contact support members, pair of contact elements, each contact element conductively secured to it a respective conductive support member, one of said conductive support members fixedly supported on said base member, the other of said conductive support members being mounted at one end thereof on said base member to permit movement of the other end thereof in relation to said fixed conductive support member,

said conductive support members being aligned on said base member so that said contact elements engage and disengage in response to movement of said movable contact support member,

insulating means for mounting one of said conductive support member on said base member in electrical insulating relationship with respect to the other of said conductive support members, a pair of terminal means, each of said conductive members conductively connected to a respective terminal means,

an element of metal oxide varistor material having a pair of spaced surface portions connected in cir cuit with said pair of terminal means.

2. The combination of claim 1 in which said metal oxide varistor material has an alpha in excess of 10 in the current density range of 10" to l0 amperes per square centimeter.

3. The combination of claim 1 in which the spaced surface portions of said element of metal oxide varistor material are spaced toprovide a standby current flow between said conductive members which is low when normal operating voltage appears across said conductive members and when voltages in excess of normal voltage progressively appear thereacross rapidly decreasing impedance is presented by said element in accordance with the alpha of said oxide varistor material thereby limiting the variation in the voltage between said conductive support members.

4. The combination of claim '1 in which said base member is conductive and in which said one conductive support member is conductively secured thereto.

5. The combination of claim 1 in which said base member is conductive and in which said other conduc tive support member is conductively secured thereto.

6. The combination of claim 4 in which said element of metal oxide varistor material is said insulating means.

7. The combination of claim 5 in which said element of metal oxide varistor material is said insulating means.

8. The combination of claim 4 in which one surface portion of said element of metal oxide varistor material is conductively secured to one of said terminal means and the other surface portion thereof is conductively connected to said movable conductive member.

9. The combination of claim 4 in which one surface portion of said element of metal oxide varistor material is conductively connected to one of said conductive members and the other surface portion of said element of metal oxide varistor material is conductively connected to the other of said conductive members.

10. The combination of claim 4 in which said element of metal oxide varistor material has one surface portion conductively secured to a non-engaging portion of said movable contact secured to said one conductive support member and the other surface portion connected to'said movable conductive member.

11. The combination of claim 4 in which one of said conductive contact elements includes a pair of contact portions, one of said contact portions being located ad-. jacent the support end of said movable conductive member and the other of said contact portions being located remote from said support end, said one contact portion being said element of metal oxide varistor material whereby said one portion of said one contact element disengages subsequent to disengagement of said other portion thereof.

12. The combination of claim 4 in which said movable conductive support member is a pair of bifurcate parts, the contact element secured to said one conductive member including a pair of contact portions, the contact portion of one of said contact elements adapted to engage and disengage with a respective contact portion of the other of said contact elements, one of the bifurcate parts of said movable conductive support member being biased to effect disengagement of the contacts associated therewith subsequent to disengagement of the contacts associated with said other bifurcate part, one of the contact portions associated with said one bifurcate part being said element of oxide varistor material.

13. The combination of claim 5 in which said element of metal oxide varistor material has one surface portion conductively secured to said one conductive member and the other surface portion thereof conductively secured to said base member.

14. The combination of claim 5 in which said element of metal oxide varistor material has one surface portion conductively secured to a non-engaging portion of the contact connected to said one conductive member and said other surface portion connected to said other conductive member.

' 15. The combination of claim 5 in which said element of metal oxide varistor material has one surface secured to said plate and the other surface conductively secured to one of said terminal means, the other terminal means conductively connected to said base member. 

2. The combination of claim 1 in which said metal oxide varistor material has an alpha in excess of 10 in the current density range of 10 3 to 102 amperes per square centimeter.
 3. The combination of claim 1 in which the spaced surface portions of said element of metal oxide varistor material are spaced to provide a standby current flow between said conductive memberS which is low when normal operating voltage appears across said conductive members and when voltages in excess of normal voltage progressively appear thereacross rapidly decreasing impedance is presented by said element in accordance with the alpha of said oxide varistor material thereby limiting the variation in the voltage between said conductive support members.
 4. The combination of claim 1 in which said base member is conductive and in which said one conductive support member is conductively secured thereto.
 5. The combination of claim 1 in which said base member is conductive and in which said other conductive support member is conductively secured thereto.
 6. The combination of claim 4 in which said element of metal oxide varistor material is said insulating means.
 7. The combination of claim 5 in which said element of metal oxide varistor material is said insulating means.
 8. The combination of claim 4 in which one surface portion of said element of metal oxide varistor material is conductively secured to one of said terminal means and the other surface portion thereof is conductively connected to said movable conductive member.
 9. The combination of claim 4 in which one surface portion of said element of metal oxide varistor material is conductively connected to one of said conductive members and the other surface portion of said element of metal oxide varistor material is conductively connected to the other of said conductive members.
 10. The combination of claim 4 in which said element of metal oxide varistor material has one surface portion conductively secured to a non-engaging portion of said movable contact secured to said one conductive support member and the other surface portion connected to said movable conductive member.
 11. The combination of claim 4 in which one of said conductive contact elements includes a pair of contact portions, one of said contact portions being located adjacent the support end of said movable conductive member and the other of said contact portions being located remote from said support end, said one contact portion being said element of metal oxide varistor material whereby said one portion of said one contact element disengages subsequent to disengagement of said other portion thereof.
 12. The combination of claim 4 in which said movable conductive support member is a pair of bifurcate parts, the contact element secured to said one conductive member including a pair of contact portions, the contact portion of one of said contact elements adapted to engage and disengage with a respective contact portion of the other of said contact elements, one of the bifurcate parts of said movable conductive support member being biased to effect disengagement of the contacts associated therewith subsequent to disengagement of the contacts associated with said other bifurcate part, one of the contact portions associated with said one bifurcate part being said element of oxide varistor material.
 13. The combination of claim 5 in which said element of metal oxide varistor material has one surface portion conductively secured to said one conductive member and the other surface portion thereof conductively secured to said base member.
 14. The combination of claim 5 in which said element of metal oxide varistor material has one surface portion conductively secured to a non-engaging portion of the contact connected to said one conductive member and said other surface portion connected to said other conductive member.
 15. The combination of claim 5 in which said element of metal oxide varistor material has one surface secured to said plate and the other surface conductively secured to one of said terminal means, the other terminal means conductively connected to said base member. 